Meeting the energy demands of all living things involves the release of energy inherent in the structures of various nutrient and storage compounds, and the capture of this energy in a form of general biochemical utility, namely the anhydride bond energy of adenosine triphosphate. Electron transport chains are of central importance in this transduction, and cytochrome c is an essential electron carrier in all such systems. Cytochrome c from eukaryotic mitochondria has been extensively studied and its structure is known in considerable detail from X-ray crystallographic studies. However the crystal structure of cytochrome c is difficult to reconcile with certain observations relating to its structure in solution. The reactivity of the tyrosyl phenolic groups is a case in point. The proposed research has two major objectives. The first is examination of those aspects of the solution structure of cytochrome c that strongly influence the reactivity of the tyrosyl phenolic groups. This part of the work will employ O-13C-acetyl substituents of the phenolic groups as NMR-detectable probes of their surroundings. The results of previous work indicate that these O-acetyl tyrosyl derivatives of cytochrome c are essentially identical or only slightly altered in conformation and activity relative to the native protein. The second objective is to study the interaction of cytochrome c with its mitochondrial site of action and related model systems, using these O-13C-acetyl derivatives as NMR-detectable reporter groups.